Inspection of people for weapons and contraband at security checkpoints has become more common in recent years. Systems using non-ionizing radiation have been developed for such inspection. In such systems there is electronic control of an array of antennas to focus millimeter wave energy on a desired spot. The resultant reflection is collected from this focus point and processed to produce a display pixel. The focus point can be scanned across a three-dimensional volume in order to provide an image display of what is contained in the volume.
For example, WO2005017559 describes a millimeter wave scanning antenna for collecting frequency-dependent beams of millimeter wave radiation from a narrow one-dimensional field of view. The collected radiation is amplified at the collected frequencies. U.S. Pat. No. 6,965,340, US2006214833, and US2006214832 describe use of an array of antenna elements which are programmable with a phase delay. US2006214834 describes use of two phase shifts 180° apart with minimization of noise. US2007013575 in which sidelobes are suppressed. U.S. Pat. No. 7,710,326 describes an array for such systems, in which there is an active device for two antenna elements. US20050093733 describes a hybrid system with a metal detector and a millimeter wave imaging system.
In many known millimeter wave imaging systems the switching of the focus point is implemented by changing control signals to the antenna array, in which one control signal to each antenna changes the antenna phase shift between 0° and 180°. The switching of the control signal results in the creation of unwanted electromagnetic radiation, particularly where there is high-speed switching and a large number of antennas, of the order of for example forty thousand). This radiation is quite significant when the entire antenna array is taken into account.
The invention is directed towards reducing the electromagnetic radiation across the antenna array whilst maintaining the image quality.